HAL Tutorial

Table of Contents

JavaScript must be enabled in your browser to display the table of contents.

1. Introduction

Configuration moves from theory to device — HAL device that is. For
those who have had just a bit of computer programming, this section is
the Hello World of the HAL. Halrun can be used to create a working
system. It is a command line or text file tool for configuration and
tuning. The following examples illustrate its setup and operation.

1.1. Notation

Terminal commands are shown without the system prompt unless you are
running HAL. The terminal window is in Applications/Accessories
from the main Ubuntu menu bar.

Terminal Command Example

me@computer:~linuxcnc$ halrun
(will be shown like the following line)
halrun
(the halcmd: prompt will be shown when running HAL)
halcmd: loadrt debounce
halcmd: show pin

1.2. Tab-completion

Your version of halcmd may include tab-completion. Instead of
completing file names as a shell does, it completes commands with HAL
identifiers. You will have to type enough letters for a unique match.
Try pressing tab after starting a HAL command:

1.3. The RTAPI environment

RTAPI stands for Real Time Application Programming Interface. Many HAL
components work in realtime, and all HAL components store data in
shared memory so realtime components can access it. Normal Linux does
not support realtime programming or the type of shared memory that HAL
needs. Fortunately there are realtime operating systems (RTOS’s) that
provide the necessary extensions to Linux. Unfortunately, each RTOS
does things a little differently.

To address these differences, the LinuxCNC team came up with RTAPI, which
provides a consistent way for programs to talk to the RTOS. If you are
a programmer who wants to work on the internals of LinuxCNC, you may want to
study linuxcnc/src/rtapi/rtapi.h to understand the API. But if you are a
normal person all you need to
know about RTAPI is that it (and the RTOS) needs to be loaded into the
memory of your computer before you do anything with HAL.

2. A Simple Example

2.1. Loading a component

For this tutorial, we are going to assume that you have successfully
installed the Live CD and, if using a RIP [Run In Place, when the
source files have been downloaded to a user directory.], invoked the
rip-environment script to prepare your shell.
In that case, all you need to do is
load the required RTOS and RTAPI modules into memory. Just run the
following command from a terminal window:

Loading HAL

cd linuxcnc
halrun
halcmd:

With the realtime OS and RTAPI loaded, we can move into the first
example. Notice that the prompt is now shown as halcmd:.
This is because subsequent commands will be interpreted as HAL commands,
not shell commands.

For the first example, we will use a HAL component called siggen,
which is a simple signal generator. A complete description of the
siggen component can be found in the Siggen section of
this Manual.
It is a realtime component, implemented as a Linux kernel module. To
load siggen use the HAL command loadrt.

Loading siggen

halcmd: loadrt siggen

2.2. Examining the HAL

Now that the module is loaded, it is time to introduce halcmd , the
command line tool used to configure the HAL. This tutorial will
introduce some halcmd features, for a more complete description try
man halcmd, or see the reference in Hal Commands
section of this document. The first
halcmd feature is the show command. This command displays information
about the current state of the HAL. To show all installed components:

Since halcmd itself is a HAL component, it will always show up in
the list. The number after halcmd in the component list is the process ID.
It is possible to run more than one copy of halcmd at the same time (in
different windows for example), so the PID is added to the end of the
name to make it unique. The list also shows the siggen component
that we installed in the previous step. The RT under Type indicates
that siggen is a realtime component. The User under Type indicates
it is a user space component.

This command displays all of the pins in the current HAL. A complex system
could have dozens or hundreds of pins. But right now there are only
nine pins. Of these pins eight are floating point and one is bit (boolean).
Six carry data out of the siggen component and three are used to transfer
settings into the component. Since we have not yet executed the code
contained within the component, some the pins have a value of zero.

The show param command shows all the parameters in the HAL. Right now
each parameter has the default value it was given when the component
was loaded. Note the column labeled Dir. The parameters labeled -W
are writable ones that are never changed by the component itself,
instead they are meant to be changed by the user to control the
component. We will see how to do this later. Parameters labeled R-
are read only parameters. They can be changed only by the component.
Finally, parameter labeled RW are read-write parameters. That means
that they are changed by the
component, but can also be changed by the user. Note: the parameters
siggen.0.update.time and siggen.0.update.tmax are for debugging
purposes, and won’t be covered in this section.

Most realtime components export one or more functions to actually run
the realtime code they contain. Let’s see what function(s) siggen
exported:

The siggen component exported a single function. It requires floating
point. It is not currently linked to any threads, so users is
zero.

2.3. Making realtime code run

To actually run the code contained in the function siggen.0.update,
we need a realtime thread. The component called threads that is used
to create a new thread. Lets create a thread called test-thread with
a period of 1 ms (1,000 us or 1,000,000 ns):

It did. The period is not exactly 1,000,000 ns because of hardware
limitations, but we have a thread that runs at approximately the
correct rate, and which can handle floating point functions. The next
step is to connect the function to the thread:

Add Function

halcmd: addf siggen.0.update test-thread

Up till now, we’ve been using halcmd only to look at the HAL.
However, this time we used the addf (add function) command to
actually change something in the HAL. We
told halcmd to add the function siggen.0.update to the thread
test-thread, and if we look at the thread list again, we see that it
succeeded:

There is one more step needed before the siggen component starts
generating signals. When the HAL is first started,
the thread(s) are not actually running. This is to allow you to
completely configure the system before the realtime code starts. Once
you are happy with the configuration, you can start the realtime code
like this:

We did two show pin commands in quick succession, and you can see
that the outputs are no longer zero.
The sine, cosine, sawtooth, and triangle outputs are
changing constantly. The square output is also working, however it
simply switches from +1.0 to -1.0 every cycle.

2.4. Changing Parameters

The real power of HAL is that you can change things. For example, we
can use the setp command to set the value of a parameter. Let’s
change the amplitude
of the signal generator from 1.0 to 5.0:

Note that the value of parameter siggen.0.amplitude has changed to
5, and that the pins now have larger values.

2.5. Saving the HAL configuration

Most of what we have done with halcmd so far has simply been viewing
things with the show command. However two of the commands actually
changed things. As we
design more complex systems with HAL, we will use many commands to
configure things just the way we want them. HAL has the memory of an
elephant, and will retain that configuration until we shut it down. But
what about next time? We don’t want to manually enter a bunch of
commands every time we want to use the system. We can save the
configuration of the entire HAL with a single command:

The output of the save command is a sequence of HAL commands. If
you start with an empty
HAL and run all these commands, you will get the configuration that
existed when the save command was issued. To save these commands
for later use, we simply
redirect the output to a file:

Save to a file

halcmd: save all saved.hal

2.6. Exiting halrun

When you’re finished with your HAL session type exit at the halcmd:
prompt. This will return you to the system prompt and close down the HAL
session. Do not simply close the terminal window without shutting down
the HAL session.

Exit HAL

halcmd: exit

2.7. Restoring the HAL configuration

To restore the HAL configuration stored in saved.hal, we need to
execute all of those HAL commands. To do that, we use -f <file name>
which reads commands from a file, and -I (upper case i) which shows
the halcmd prompt after executing the commands:

Run a Saved File

halrun -I -f saved.hal

Notice that there is not a start command in saved.hal. It’s
necessary to issue it again (or edit saved.hal to add it there).

2.8. Removing HAL from memory

If an unexpected shut down of a HAL session occurs you might have to
unload HAL before another session can begin. To do this type the
following command in a terminal window.

Removing HAL

halrun -U

3. Halmeter

You can build very complex HAL systems without ever using a graphical
interface. However there is something satisfying about seeing the
result of your work. The first and simplest GUI tool for the HAL is
halmeter. It is a very simple program that is the HAL equivalent of the
handy Fluke multimeter (or Simpson analog meter for the old timers).

We will use the siggen component again to check out halmeter. If you
just finished the previous example, then you can load siggen using the
saved file. If not, we can load it just like we did before:

At this point we have the siggen component loaded and running. It’s
time to start halmeter.

Starting Halmeter

halcmd: loadusr halmeter

The first window you will see is the Select Item to Probe window.

Figure 1. Halmeter Select Window

This dialog has three tabs. The first tab displays all of the HAL pins
in the system. The second one displays all the signals, and the third
displays all the parameters. We would like to look at the pin
siggen.0.cosine first, so click on it then click the Close button.
The probe
selection dialog will close, and the meter looks something like the
following figure.

Figure 2. Halmeter

To change what the meter displays press the Select button which
brings back the Select Item to Probe window.

You should see the value changing as siggen generates its cosine wave.
Halmeter refreshes its display about 5 times per second.

To shut down halmeter, just click the exit button.

If you want to look at more than one pin, signal, or parameter at a
time, you can just start more halmeters. The halmeter window was
intentionally made very small so you could have a lot of them on the
screen at once.

4. Stepgen Example

Up till now we have only loaded one HAL component. But the whole idea
behind the HAL is to allow you to load and connect a number of simple
components to make up a complex system. The next example will use two
components.

Before we can begin building this new example, we want to start with a
clean slate. If you just finished one of the previous examples, we need
to remove the all components and reload the RTAPI and HAL libraries.

halcmd: exit

4.1. Installing the components

Now we are going to load the step pulse generator component. For a
detailed description of this component refer to the stepgen section of the
Integrator Manual. In this example we will use the velocity control
type of stepgen. For now, we can skip the details, and just run the
following commands.

The first command loads two step generators, both configured to
generate stepping type 0. The second command loads our old friend
siggen, and the third one creates two threads, a fast one with a period
of 50 microseconds and a slow one with a period of 1 millisecond. The fast
thread doesn’t support floating point functions.

As before, we can use halcmd show to take a look at the HAL. This
time we have a lot more pins and parameters than before:

4.2. Connecting pins with signals

What we have is two step pulse generators, and a signal generator. Now
it is time to create some HAL signals to connect the two components. We
are going to pretend that the two step pulse generators are driving the
X and Y axis of a machine. We want to move the table in circles. To do
this, we will send a cosine signal to the X axis, and a sine signal to
the Y axis. The siggen module creates the sine and cosine, but we need
wires to connect the modules together. In the HAL, wires are called
signals. We need to create two of them. We can call them anything we
want, for this example they will be X-vel and Y-vel. The signal
X-vel is intended to run from the cosine output of the signal
generator to the velocity input of the first step pulse generator.
The first step is to connect the signal to the signal generator output.
To connect a signal to a pin we use the net command.

When a signal is connected to one or more pins, the show command lists
the pins immediately following the signal name. The arrow shows the
direction of data flow - in this case, data flows from pin
siggen.0.cosine to signal X-vel. Now let’s connect the X-vel to
the velocity input of a step pulse generator.

halcmd: net X-vel => stepgen.0.velocity-cmd

We can also connect up the Y axis signal Y-vel. It is intended to
run from the sine output of the signal generator
to the input of the second step pulse generator. The following command
accomplishes in one line what two net commands accomplished for
X-vel.

halcmd: net Y-vel siggen.0.sine => stepgen.1.velocity-cmd

Now let’s take a final look at the signals and the pins connected to
them.

The show sig command makes it clear exactly how data flows through
the HAL. For example, the X-vel signal comes from pin
siggen.0.cosine, and goes to pin stepgen.0.velocity-cmd.

4.3. Setting up realtime execution - threads and functions

Thinking about data flowing through wires makes pins and signals
fairly easy to understand. Threads and functions are a little more
difficult. Functions contain the computer instructions that actually
get things done. Thread are the method used to make those instructions
run when they are needed. First let’s look at the functions available
to us.

In general, you will have to refer to the documentation for each
component to see what its functions do. In this case, the function
siggen.0.update is used to update the outputs of the signal
generator. Every time it is executed, it calculates the values of
the sine, cosine, triangle, and square outputs. To make smooth
signals, it needs to run at specific intervals.

The other three functions are related to the step pulse generators.

The first one, stepgen.capture_position, is used for position
feedback. It captures the value of an internal
counter that counts the step pulses as they are generated. Assuming no
missed steps, this counter indicates the position of the motor.

The main function for the step pulse generator is
stepgen.make_pulses. Every time make_pulses runs it decides if it
is time to take a step, and if so sets the
outputs accordingly. For smooth step pulses, it should run as
frequently as possible. Because it needs to run so fast, make_pulses
is highly optimized and performs only a few calculations. Unlike the
others, it does not need floating point math.

The last function, stepgen.update-freq, is responsible for doing
scaling and some other calculations that need to be performed
only when the frequency command changes.

What this means for our example is that we want to run
siggen.0.update at a moderate rate to calculate the sine and cosine
values. Immediately after we run siggen.0.update, we want to run
stepgen.update_freq to load the new values into the step pulse
generator. Finally we need
to run stepgen.make_pulses as fast as possible for smooth pulses.
Because we don’t use position
feedback, we don’t need to run stepgen.capture_position at all.

We run functions by adding them to threads. Each thread runs at a
specific rate. Let’s see what threads we have available.

The two threads were created when we loaded threads. The first one,
slow, runs every millisecond, and is capable of running floating
point functions. We will use it for siggen.0.update and
stepgen.update_freq. The second thread is fast, which runs every
50 microseconds, and does not support floating point.
We will use it for stepgen.make_pulses. To connect the
functions to the proper thread, we use the addf command.
We specify the function first, followed by the thread.

Now each thread is followed by the names of the functions, in the
order in which the functions will run.

4.4. Setting parameters

We are almost ready to start our HAL system. However we still need to
adjust a few parameters. By default, the siggen component generates
signals that swing from +1 to -1. For our example that is fine, we want
the table speed to vary from +1 to -1 inches per second. However the
scaling of the step pulse generator isn’t quite right. By default, it
generates an output frequency of 1 step per second with an input of
1.000. It is unlikely that one step per second will give us one inch
per second of table movement. Let’s assume instead that we have a 5
turn per inch leadscrew, connected to a 200 step per rev stepper with
10x microstepping. So it takes 2000 steps for one revolution of the
screw, and 5 revolutions to travel one inch. that means the overall
scaling is 10000 steps per inch. We need to multiply the velocity input
to the step pulse generator by 10000 to get the proper output. That is
exactly what the parameter stepgen.n.velocity-scale is for. In this
case, both the X and Y axis have the same scaling, so
we set the scaling parameters for both to 10000.

This velocity scaling means that when the pin stepgen.0.velocity-cmd
is 1.000, the step generator will generate 10000 pulses per second
(10KHz). With the motor and leadscrew described above, that will result
in the axis moving at exactly 1.000 inches per second. This illustrates
a key HAL concept - things like scaling are done at the lowest possible
level, in this case in the step pulse generator. The internal signal
X-vel is the velocity of the table in inches per second, and other
components such as siggen don’t know (or care) about the scaling at
all. If we changed the leadscrew, or motor, we would change only the
scaling parameter of the step pulse generator.

4.5. Run it!

We now have everything configured and are ready to start it up. Just
like in the first example, we use the start command.

halcmd: start

Although nothing appears to happen, inside the computer the step pulse
generator is cranking out step pulses, varying from 10KHz forward to
10KHz reverse and back again every second. Later in this tutorial we’ll
see how to bring those internal signals out to run motors in the real
world, but first we want to look at them and see what is happening.

5. Halscope

The previous example generates some very interesting signals. But much
of what happens is far too fast to see with halmeter. To take a closer
look at what is going on inside the HAL, we want an oscilloscope.
Fortunately HAL has one, called halscope.

Halscope has two parts - a realtime part that is loaded as a kernel
module, and a user part that supplies the GUI and display. However, you
don’t need to worry about this, because the userspace portion will
automatically request that the realtime part be loaded. Also notice
the first time you run halscope in a directory it gives a benign
message that the file autosave.halscope could not be opened.

Starting Halscope

halcmd: loadusr halscope
halcmd: halscope: config file 'autosave.halscope' could not be opened

The scope GUI window will open, immediately followed by a
Realtime function not linked dialog that looks like the following
figure.

Figure 3. Realtime function not linked dialog

This dialog is where you set the sampling rate for the oscilloscope.
For now we want to sample once per millisecond, so click on the 989 us
thread slow and leave the multiplier at 1. We will also leave the
record length at 4000 samples, so that we can use up to four channels
at one time. When you select a thread and then click OK, the dialog
disappears, and the scope window looks something like the following
figure.

Figure 4. Initial scope window

5.1. Hooking up the scope probes

At this point, Halscope is ready to use. We have already selected a
sample rate and record length, so the next step is to decide what to
look at. This is equivalent to hooking virtual scope probes to the
HAL. Halscope has 16 channels, but the number you can use at any one
time depends on the record length - more channels means shorter
records, since the memory available for the record is fixed at
approximately 16,000 samples.

The channel buttons run across the bottom of the halscope screen.
Click button 1, and you will see the Select Channel Source dialog
as shown in the following figure. This dialog is very similar to the
one used by Halmeter. We would like to look at the signals we defined
earlier, so we click on the Signals tab, and the dialog displays all
of the signals in the HAL (only two for this example).

Figure 5. Select Channel Source

To choose a signal, just click on it. In this case, we want channel 1
to display the signal X-vel. Click on the Signals tab then click on
X-vel and the dialog closes and the channel is now selected.

Figure 6. Select Signal

The channel 1 button is pressed in, and channel number 1 and the name
X-vel appear below the row of buttons. That display always indicates
the selected channel - you can have many channels on the screen, but
the selected one is highlighted, and the various controls like vertical
position and scale always work on the selected one.

Figure 7. Halscope

To add a signal to channel 2, click the 2 button. When the dialog
pops up, click the Signals tab, then click on Y-vel. We also want
to look at the square and triangle wave outputs. There are no signals
connected to those pins, so we use the Pins tab instead. For channel
3, select siggen.0.triangle and for channel 4, select
siggen.0.square.

5.2. Capturing our first waveforms

Now that we have several probes hooked to the HAL, it’s time to
capture some waveforms. To start the scope, click the Normal button
in the Run Mode section of the screen (upper right). Since we have a
4000 sample record length, and are acquiring 1000 samples per second,
it will take halscope about 2 seconds to fill half of its buffer.
During that time a progress bar just above the main screen will show
the buffer filling. Once the buffer is half full, the scope waits for a
trigger. Since we haven’t configured one yet, it will wait forever. To
manually trigger it, click the Force button in the Trigger section
at the top right. You should see the remainder of the buffer fill, then
the screen will display the captured waveforms. The result will look
something like the following figure.

Figure 8. Captured Waveforms

The Selected Channel box at the bottom tells you that the purple
trace is the currently selected one, channel 4, which is displaying the
value of the pin siggen.0.square. Try clicking channel buttons 1
through 3 to highlight the other three traces.

5.3. Vertical Adjustments

The traces are rather hard to distinguish since all four are on top of
each other. To fix this, we use the Vertical controls in the box to
the right of the screen. These controls act on the currently selected
channel. When adjusting the gain, notice that it covers a huge range -
unlike a real scope, this one can display signals ranging from very
tiny (pico-units) to very large (Tera-units). The position control
moves the displayed trace up and down over the height of the screen
only. For larger adjustments the offset button should be used.

Figure 9. Vertical Adjustment

5.4. Triggering

Using the Force button is a rather unsatisfying way to trigger the
scope. To set up real triggering, click on the Source button at the
bottom right. It will pop up the Trigger Source dialog, which is
simply a list of all the probes that are currently connected. Select a
probe to use for triggering by clicking on it. For this example we will
use channel 3, the triangle wave as shown in the following figure.

Figure 10. Trigger Source Dialog

After setting the trigger source, you can adjust the trigger level and
trigger position using the sliders in the Trigger box along the right
edge. The level can be adjusted from the top to the bottom of the
screen, and is displayed below the sliders. The position is the
location of the trigger point within the overall record. With the
slider all the way down, the trigger point is at the end of the record,
and halscope displays what happened before the trigger point. When the
slider is all the way up, the trigger point is at the beginning of the
record, displaying what happened after it was triggered. The trigger
point is visible as a vertical line in the progress box above the
screen. The trigger polarity can be changed by clicking the button just
below the trigger level display.

Now that we have adjusted the vertical controls and triggering, the
scope display looks something like the following figure.

Figure 11. Waveforms with Triggering

5.5. Horizontal Adjustments

To look closely at part of a waveform, you can use the zoom slider at
the top of the screen to expand the waveforms horizontally, and the
position slider to determine which part of the zoomed waveform is
visible. However, sometimes simply expanding the waveforms isn’t enough
and you need to increase the sampling rate. For example, we would like
to look at the actual step pulses that are being generated in our
example. Since the step pulses may be only 50 us long, sampling at 1KHz
isn’t fast enough. To change the sample rate, click on the button that
displays the number of samples and sample rate to bring up the Select
Sample Rate dialog, figure . For this example, we will click on the
50 us thread, fast, which gives us a sample rate of about 20KHz. Now
instead of displaying about 4 seconds worth of data, one record is 4000
samples at 20KHz, or about 0.20 seconds.

Figure 12. Sample Rate Dialog

5.6. More Channels

Now let’s look at the step pulses. Halscope has 16 channels, but for
this example we are using only 4 at a time. Before we select any more
channels, we need to turn off a couple. Click on the channel 2 button,
then click the Chan Off button at the bottom of the Vertical box.
Then click on channel 3, turn if off, and do the same for channel 4.
Even though the channels are turned off, they still remember what they
are connected to, and in fact we will continue to use channel 3 as the
trigger source. To add new channels, select channel 5, and choose pin
stepgen.0.dir, then channel 6, and select stepgen.0.step. Then
click run mode Normal to start the scope, and adjust the horizontal
zoom to 5 ms per division. You should see the step pulses slow down as
the velocity command (channel 1) approaches zero, then the direction
pin changes state and the step pulses speed up again. You might want to
increase the gain on channel 1 to about 20 milli per division to better see
the change in the velocity command. The result should look like the
following figure.

Figure 13. Step Pulses

5.7. More samples

If you want to record more samples at once, restart realtime and load
halscope with a numeric argument which indicates the number of samples
you want to capture.

halcmd: loadusr halscope 80000

If the scope_rt component was not already loaded, halscope will
load it and request 80000 total samples, so that when sampling
4 channels at a time there will be 20000 samples per channel.
(If scope_rt was already loaded, the numeric argument to
halscope will have no effect).